The evidence that Iraq was trying to buy yellowcake...
Christopher Hitchens references a Financial Post article by Mark Huband. Here is a link to that article.
Human intelligence gathered in Italy and Africa more than three years before the Iraq war had shown Niger officials referring to possible illicit uranium deals with at least five countries, including Iraq.
This intelligence provided clues about plans by Libya and Iran to develop their undeclared nuclear programmes. Niger officials were also discussing sales to North Korea and China of uranium ore or the "yellow cake" refined from it: the raw materials that can be progressively enriched to make nuclear bombs.
The raw intelligence on the negotiations included indications that Libya was investing in Niger's uranium industry to prop it up at a time when demand had fallen, and that sales to Iraq were just a part of the clandestine export plan.
These secret exports would allow countries with undeclared nuclear programmes to build up uranium stockpiles.
One nuclear counter-proliferation expert told the FT: "If I am going to make a bomb, I am not going to use the uranium that I have declared. I am going to use what I acquire clandestinely, if I am going to keep the programme hidden."
This may have been the method being used by Libya before it agreed last December to abandon its secret nuclear programme. According to the IAEA, there are 2,600 tonnes of refined uranium ore - "yellow cake" - in Libya.
However, less than 1,500 tonnes of it is accounted for in Niger records, even though Niger was Libya's main supplier.
Information gathered in 1999-2001 suggested that the uranium sold illicitly would be extracted from mines in Niger that had been abandoned as uneconomic by the two French-owned mining companies - Cominak and Somair, both of which are owned by the mining giant Cogema - operating in Niger.
"Mines can be abandoned by Cogema when they become unproductive. This doesn't mean that people near the mines can't keep on extracting," a senior European counter-proliferation official said.
He added that there was no evidence the companies were aware of the plans for illicit mining.
When the intelligence gathered in 1999-2001 was thrown into the diplomatic maelstrom that preceded the US-led invasion of Iraq, it took on new significance. Several services contributed to the picture.
The Italians, looking for corroboration but lacking the global reach of the CIA or the UK intelligence service MI6, passed information to the US in 2001 and to the UK in 2002.
The UK eavesdropping centre GCHQ had intercepted communications suggesting Iraq was seeking clandestine uranium supplies, as had the French intelligence service.
The Italian intelligence was not incorporated in detail into the assessments of the CIA, which seeks to use such information only when it is gathered from its own sources rather than as a result of liaison with foreign intelligence services.
But five months after receiving it, the US sent former ambassador Joseph Wilson to Niger to assess the credibility of separate US intelligence information that suggested Iraq had approached Niger.
Mr Wilson was critical of the Bush administration's use of secret intelligence, and has since charged that the White House sought to intimidate him by leaking the identity of his wife, Valerie Plame, as a CIA agent.
But Mr Wilson also stated in his account of the visit that Mohamed Sayeed al-Sahaf, Iraq's former information minister, was identified to him by a Niger official as having sought to discuss trade with Niger.
As Niger's other main export is goats, some intelligence officials have surmised uranium was what Mr Sahaf was referring to.